I wonder what my favorite is...guess.

It's Terra in case my avatar fails somehow.

I'm not sure I want to rate the others. They can all beat Kefka at level 99 with the exception of Umaro though I've never tried and won't.

In no particular order Edgar, Setzer, and Celes had means to deal with the empire overall besides Terra who is compeletly important to the entire theme of the series.

The others are kinda tag-alongs. They did advance the plot in some way. Gogo is probably the least found character anyway and is the least neccessary to completing the game.

But even I've solo'd the game before. It depends what you want out of it.